The present invention relates to on-line GC transfer techniques, meaning methods wherein a group of chromatographic peaks resulting from elution of a complex chemical mixture on a first GC column is transferred automatically to a second GC column in a controlled manner, or eventually to a detection apparatus, namely an olfactometric detection means.
Chromatographic analysis of complex chemical samples is based on the principle that, as the chemical sample flows along a chromatography column, each chemical is separated into a band and possibly detected in the form of a peak, thus allowing separation and identification of each ingredient in the chemical mixture which constitutes the sample. Ideally, each of the chemicals is separated into a discrete band but, in practice, it is common that several ingredients elute at very close times thus producing broad and/or overlapping bands. In an attempt to obtain separate bands and thus complete separation and identification of all the ingredients, such broad or overlapping bands may be resolved by passing them (i.e. the part of the sample eluting from the first column at the time corresponding to the band) through a second column having different chemical or physical characteristics from those of the first column, thus allowing multidimensional resolution of the GC spectrum.
Multidimensional GC is a well-known technique that has been developed early after the invention of gas chromatography. The controlled transfer of analytes from the first to the second column is a critical issue.
There are basically two known ways of carrying out this transfer. In the most recent one, the whole GC profile obtained after elution of the chemical sample fluid stream out of the first column is transferred to the second column, thermal modulation means, namely cryo-control, being provided between the two columns to improve the sensitivity of the peaks eluted out of the second column—we are typically in the realm of the so-called GCxGC or comprehensive gas chromatography field.
In all these prior known methods which use cryo-control of the analyte transfer between columns, the analytes eluting from the first column are continuously trapped and automatically re-injected into a second column at a given frequency, without possible control in the injection rate along the chromatographic run.
Due to the frequency of the transfer pulses (typically 3-5 sec), peaks must elute through the second column in the same time frame as that at which they elute from the first column. As a result, the second column must be a fast-GC column with a smaller inner diameter (typically 0.05-0.10 mm) and considerably shorter length than the first column, otherwise peak overlap may occur.
The fast elution from the second column thus requires a high sampling rate of the detector, which means that conventional detectors such as quadrupole mass spectrometer (MS), infrared (IR) detectors cannot be used.
Moreover, it also prevents optimal detection in applications suited to the fragrance industry for example, where olfactometric evaluation by a perfumer of the peaks as they elute requires high resolution and sufficient time separation between the peaks to allow detection and evaluation by the human nose of the chemical or chemicals of interest.
However, all such known GCxGC interfaces have been designed and automated to sequentially transfer the all chromatogram from the first to the second column and they are not suitable for MDGC.
To specifically re-analyze a given zone of the first chromatogram, a “targeted GC” mode has also been proposed (see for example P. J. Marriott et al., J.Chromatogr. 2000, 866, 203-212; P. J. Mariott, WO 98/21574). Using the GCxGC configuration, the target zone is cryo-trapped and transferred into the second column. This still requires a fast-GC analysis to elute the trapped zone in a few seconds in the second dimension, as the rate of retention of peaks eluting before and after the trapped zone remains unchanged. Therefore, the same drawbacks as those previously cited result.
Thus, most of the known multidimensional GC techniques deal with a second method which achieves an on-line “heart-cut”, i.e. only some peaks eluted from the first column are transferred to the second one, while others are vented. Such methods fall in the category of the so-called multidimensional gas chromatography (MDGC) techniques.
Alternative techniques, such as the intermediate trapping of analytes in a sorbent and their subsequent desorption in a second column (see for example the articles of K. A. Krok et al. in J.Chromatogr. 1994, 678, 265-277 or Anal.Chem. 1993, 65, 1012-1016), require far longer analysis times and sophisticated hardware and they do not therefore compete with the method and apparatus which are the object of the present invention.
The so-called “heart-cut” in prior art MDGC can be achieved via two means: with a valve, or with a pneumatic switcher.
A valve is the simplest interfacing as no pressure or flow control is required when two columns of the same diameter are used (see for example L. Mondello et al. in J.Chromatogr.Sci. 1998, 36, 201-209 or the disclosure in U.S. Pat. No. 5,492,555 to M. R. Strunk et al.).
However, valves can interact with the sample, in particular when the mixtures to be analyzed contain labile components (e.g. sulfur derivatives) susceptible of being degraded by the valve metallic material, namely stainless steel. Other compounds (e.g. carboxylic acids, amines) are prone to adsorption on the stainless steel surface of valves. Such phenomena cause memory effects susceptible of being prejudicial to the analysis (see, for example, B. M. Gordon et al. in J.Chromatogr. Sci. 1985, 23, 1-10).
Pneumatic switching has been proposed by D. R. Deans (see for example Chromatogr. 1968, 1, 18-21 and 1971, 4, 279-285) which avoids passing analytes through a valve.
However, pneumatic switching requires an accurate flow control of the pressure between the two columns. This must be achieved using a make-up gas and e.g. electronic mass flow controllers. Such sophistication makes the optimization of analytical parameters more complicated and increases instrument cost. Moreover, pneumatic switching may cause some peak broadening.
The use of thermal modulation means, namely a cold trap, has also been proposed to re-focus the heart-cut peak after the pneumatic switching means (A. Hagman; S. Jacobsson in Journal of High Resolution Chromatography and Chromatography Communications 1985, 8, 332-336). Then, an additional means to quickly heat and re-inject the trapped peak is required, but again this increases the complexity of the system.
Finally, a “loop modulator”, wherein the inlet and outlet of the modulator tube pass in front of a gas jet alternatively supplied with a cold and hot fluid, has also been proposed in International patent application WO 03/82427. The chromatographic column or modulation tube has a loop structure and it allows the measurement of the carrier gas velocity through the modulator tube. It mainly claims to shorten the peak width of modulated peaks.
Although the invention described in this prior art document is based on multi-stage thermal modulation of chemical substances admixed with a carrier gas and flowing through a tube, it still does not provide means to control, in a simple manner, the rate at which successive targeted analytes zones, which are cryo-trapped as they elute from the first column, reach the second column to be eluted there-through, detected and possibly evaluated by a perfumer as they come out of the second column.
In short, none of the prior known chromatographic methods and devices involving automatic transfer of analytes from a first separation column to a second separation column, or to a detection device such as a physical or biological detector, in particular a human nose, comprising a thermal modulator in the analyte transfer line, allows the control of the speed at which a selected peak or group of peaks, which it is desired to completely separate, elutes through the second column or reaches the detector.
The present invention aims at solving this problem.